(Not Applicable)
The present invention relates in general to the production of mold-cast structures, and in particular to a method for controlling solidification rate and pore formation of a molten liquid metal within a mold casting chamber by measuring and regulating soluble-gas pressure within the chamber and temperature and/or heat flow change at a plurality of chamber sites to thereby fabricate a solid porous metal structure having known characteristics produced as a result of such chosen pressure and temperature regulation.
Production of numerous products is accomplished through employment of mold fabrication technology whereby hot liquid material constituting the substance of a finished product is placed within a mold chamber shaped in the form of the desired final product and thereafter cooled to solidify and yield the finished product. Eligible materials for moldable products generally must be able to withstand heating to a flowable liquid state without untoward breakdown of components and to ultimately cool after formation into an acceptable product. Two typical families of such materials are found in plastics and metals, thereby resulting in various plastic polymers and feasibly-meltable metals being mold-formed into a myriad of products.
While the generalized steps of heating a material to melt, introducing the molten material to a mold cavity, and cooling the material to form a finished product are well known, specific procedures and methodology during these steps can significantly contribute to end product results. Thus, for example, the rate of cooling and thus solidification of particular molten metals can affect the microstructure of the finished metal structure. One prior art attempt to regulate cooling includes actual movement of a mold cavity having therein the metal through a series of decreasing temperature zones to thereby produce a general, and obviously non-precise, cooling effect over a period of time. Another prior art attempt to regulate cooling is a simple reduction of heat to the mold cavity in a non-precise manner. While solid structure formation of a molded product readily occurs through these prior art methods, the actual microstructure of the product is not standardized because consistency of cooling and therefore consistency of the solidification rate is not achieved.
In addition to forming solid structures in general, it many times is desirous to form solid structures, such as metal structures for example, that have internal porosities to thereby provide weight and structural characteristics congruent with particular product applications. One known procedure for providing pores within a mold-fabricated metal structure is to force a soluble gas such as hydrogen under pressure into molten metal, as shown for example in U.S. Pat. No. 5,181,549 to Shapovalov. Dissolved-gas behavior is such that its solubility decreases with decreasing temperature and decreasing pressure, thereby simultaneously responding to two separate parameters that influence activity. During cooling and/or depressurization, the dissolved gas precipitates and goes to bubbles that do not leave, but, instead, form pores. While the prior art recognizes such gas behavior in porosity formation, the prior art does not teach methodology employing precision parameter measurement followed by precision parameter adjustment for controlled structural formation.
In view of the short comings noted above, it is apparent that a need is present for a method of providing significant control over solidification rates along with internal pore formation of structures formed within a mold casting chamber. Accordingly, a primary object of the present invention is to provide a method of controlling a solidification rate of a molten liquid metal within a casting chamber of a mold while additionally controlling pore formation within the metal by continuously monitoring and adjusting pressure within the chamber and continuously monitoring and adjusting temperature values at a plurality of sites relative the casting chamber.
Another object of the present invention is to provide a method of controlling such porosity and rate of solidification wherein a microprocessor determines and accordingly regulates pressure within the chamber and temperature values at each such site in concordance with stored pressure and temperature measurements relating to respective extents of pore formation and solidification.
These and other objects of the present invention will become apparent throughout the description thereof which now follows.
The present invention is a method of fabricating a porous metal structure from a molten liquid metal within a casting chamber of a mold upon controlled cooling thereof. The method first comprises providing a stationary mold comprising a gas-pressurizable casting chamber with a heat-transferable wall having a plurality of sites each having in communication therewith a respective surface-temperature sensor for determining a respective temperature at each such site. Each site additionally includes an independently operable temperature controller for regulating each respective site temperature. The mold is provided with a gas pressure release valve for releasing gas from the casting chamber and an internal gas pressure measurement sensor for measuring chamber pressure. The method next includes providing a microprocessor comprising first a plurality of stored temperature measurements relating to respective extents of solidification of molten liquid metal at each of the plurality of stored temperature measurements, and second a plurality of stored gas pressure measurements relating to respective extents of solubilized gas molecules within the molten liquid metal for determining porosity thereof. The microprocessor is in communication with each respective surface-temperature sensor for receiving each respective temperature at each site, in communication with each respective temperature controller for selective operation thereof, in communication with said gas pressure measurement sensor for receiving pressure magnitude within the casting chamber, and in communication with the gas pressure release valve for selective operation thereof. The casting chamber is heated to a temperature sufficient to maintain the liquid metal in a molten state, and the molten liquid metal is situated within the casting chamber. A gas at least partially soluble in the molten metal is introduced thereto under pressure of a magnitude sufficient to force a sufficient quantity of solubilized gas molecules into the molten metal for forming pores upon cooling thereof to a porous metal structure. Finally, the microprocessor is activated for receiving each respective temperature at each site and pressure magnitude within the chamber, comparing each respective temperature and pressure magnitude to the stored temperature and pressure measurements, and regulating in response thereto the gas pressure release valve and each respective temperature controller for continuously maintaining a magnitude of pressure and rate of cooling within the casting chamber equal to chosen extents of porosity and solidification over a time period terminating upon fabrication of the porous metal structure.
In a second preferred embodiment, pressure control is identical to that of the first embodiment while the surface-temperature sensors are replaced with or provided in conjunction with heat flux sensors for determining a respective heat removal rate at each site. In addition to stored depressurization rates, the microprocessor includes a plurality of stored heat removal rates relating to respective extents of solidification of liquid metal at each of these stored heat removal rates. The activated microprocessor receives each respective heat removal rate at each site, compares each heat removal rate to the stored heat removal rates, compares and correlates depressurization rates, and regulates in response thereto the pressure relief valve and each respective temperature controller for continuously maintaining pore formation and cooling rate again equal to chosen extents of porosity and solidification over a time period terminating upon fabrication of the solid structure.
The methodology here defined permits precision temperature and pressure management in accord with historical parameters as reflected in algorithmic analyses and regulation via the microprocessor to achieve structure development in accord with specified product production.